To maintain clear vision the eyes must be kept reasonably still with respect to the surroundings. Doing this requires compensating for both rotations and translations of the head. Visual cues are important in this, as shown by the severe postural instabilities produced in humans by unanticipated rotations or translations of the visual surroundings. In this proposal we wish to argue that to perform these compensations the brain must be able to distinguish rotations from translations, and that it does so by means of entirely independent mechanisms. We propose to explore a behavior in which the responses to translation and rotation are robust and immediately distinguishable as a means to understanding what is required for the visual system to resolve the optic flow information into translational and rotational components. The system we propose to study is that responsible for the stabilization of the head and eyes in the chicken. We propose to use head movements as a probe for the processing of visual motion information whereby translational and rotational components are distinguished. First, we will characterize the head and eye responses to sinusoidal translational and rotational visual stimuli over a range of frequencies and velocities. Second, we will examine the effects of restricting the field of view to one eye and to restricted parts of the visual fields of both eyes on responses to translational and rotational stimuli. These experiments should reveal whether comparison of the direction of motion in different parts of the visual field is the cue used to resolve motion into rotational and translational components. Third, we will examine the dependence of normal head stability on visual inputs. Fourth, we will investigate the role of the vestibular system in these response; for this we will examine the translational and rotational behavior of animals with lesioned vestibular systems and compare them to the normal. Our preliminary work suggest that the translational response of the head is not a manifestation of the "classical" optokinetic system, but rather that a different neural system is responsible for the visual components of this behavior. Possibly this will cast light on parallel processes in humans.